Synthesis, Structure, and Properties of the Layered Oxyselenide Ba₂CuO₂Cu₂Se₂

A new layered oxyselenide, Ba₂CuO₂Cu₂Se₂, was synthesized under high-pressure and high-temperature conditions and was characterized via structural, magnetic, and transport measurements. It crystallizes into space group I4/ mmm and consists of a square lattice of [CuO₂] planes and antifluorite-type [Cu₂Se₂] layers, which are alternately stacked along the c axis. The lattice parameters are obtained as a = b = 4.0885 Å and c = 19.6887 Å. The Cu-O bond length is given by half of the lattice constant a, i.e., 2.0443 Å. Ba₂CuO₂Cu₂Se₂ is a semiconductor with a resistivity of ∼18 mΩ·cm at room temperature. No magnetic transition was found in the measured temperature range, and the Curie-Weiss temperature was obtained as -0.2 K, suggesting a very weak exchange interaction. The DFT+ U_eff calculation demonstrates that the band gap is about 0.2 eV for the supposed antiferromagnetic order, and the density of state near the top of the valence band is mainly contributed from the Se 4p electrons.